Generally described, synthetic gas, or syngas, may be produced in a gasification reactor by reacting a carbonaceous fuel in a partial oxidation process. The syngas then may be sent to a number of scrubbers for removing contaminants and fine particles therein. Specifically, the syngas may include hydrogen sulfide, ammonia, cyanides, phenols, various halogens, and particulates in the form of carbon, ash, coal, trace metals, and the like. These contaminants and fines must be disposed of in a satisfactory manner for the gasification process to be economically and environmentally viable.
A large quantity of water is required for use in the overall gasification process. Specifically, water may be used in the gasification reactor for quenching the syngas. Further, water also may be used in the scrubbers for removing the fines. The water used in the scrubbers may become what is known as “black water” because it is contaminated with carbon, ash, and the like. The black water also may contain soluble gases therein. The black water may have about five percent (5%) to about ten percent (10%) by weight of suspended solids therein and may have an initial temperature of about 300 degrees Celsius or more.
Known techniques to clean and recycle this black water generally involve the use of a flash system. Such a flash system may include a number of flash drums positioned in stages from high pressure, to low pressure, to low vacuum, and to high vacuum. The vacuum stages not only help to remove the dissolve gases, but also help to cool the black water. The black water from the flash system may be sent to a gravity settler. The fines may be removed and clear water may be recycled in the gravity settler. The solids from the gravity settler then may be sent for grinding to prepare a slurry that may be fed to the gasifier and combusted therein.
The use of flash drums up to atmospheric pressure may be economical in that no additional energy is required to remove the dissolved gases. The use of flashing in vacuuming conditions, however, generally requires large equipment with significant energy demands and maintenance requirements. Specifically, such a vacuum system requires large amounts of energy in that the flash drums remove only dissolved gases but also a significant quantity of water vapor. High cooling water consumption therefore may be required to condense the water vapor. This water and energy consumption is considered a parasitical loss on the power plant as a whole.
There is thus a desire for improved black water treatment and recycling system and methods. Preferably, such improved systems and methods may treat and recycle black water produced by a gasifier and the like without requiring the large devices and high energy demands found in known flash systems using vacuum conditions.